Method for manufacturing boundary acoustic wave device

ABSTRACT

A manufacturing method for a boundary acoustic wave device is capable of certainly providing the boundary acoustic wave device with desired target frequency characteristics. The manufacturing method for the boundary acoustic wave device includes a process for preparing a laminated body that includes a first medium, a second medium laminated on the first medium, and an IDT electrode that is disposed at an interface between the first and second media, and a process for implanting ions from an outer portion of the second medium and adjusting a frequency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a manufacturing method for a boundaryacoustic wave device preferably for use in a resonator, a bandpassfilter, or the like, for example, and the boundary acoustic wave device,and also relates to a manufacturing method for a boundary acoustic wavedevice, which includes a process for adjustment of a frequency, and theboundary acoustic wave device, in more detail.

2. Description of the Related Art

In recent years, in place of surface acoustic wave devices, boundaryacoustic wave devices have been drawing attention. In the boundaryacoustic wave device, boundary acoustic waves propagate through aboundary between first and second media made of solid substances.Accordingly, in the boundary acoustic wave device, compared with asurface acoustic wave device, it is possible to promote the downsizingof the device. In addition, it is possible to achieve the simplificationof a package structure.

When the boundary acoustic wave device is used as a filter or aresonator, it is necessary to set the frequency characteristics thereofwith a high degree of accuracy. However, in the boundary acoustic wavedevice, an interdigital transducer (IDT) electrode exists at theboundary of the first and second media. Accordingly, unlike the surfaceacoustic wave device, it is difficult to adjust the frequency thereof.

In WO2008/062639 A1, an example of a method of adjusting a frequency ofthe boundary acoustic wave device is disclosed. As illustrated in FIG.12, an IDT electrode 103 is disposed at the boundary of a first medium101 and a second medium 102. The second medium 102 includes a mediumlayer 102 a and a medium layer 102 b, and a reforming medium layer 102 cis disposed between the medium layers 102 a and 102 b.

In WO2008/062639 A1, the outer side portion of the second medium 102 isirradiated with laser light as illustrated by an arrow. Owing to theirradiation of the laser light, the reforming medium layer 102 c isheated, and metal included in the reforming medium layer 102 c isdiffused within the second medium 102. It is supposed that, owing to thediffusion, a reforming portion is formed and hence it is possible toenhance adjustment of a frequency.

In the boundary acoustic wave device, the propagation energy of theboundary acoustic waves is concentrated into a portion near the boundaryof the first medium 101 and the second medium 102. Accordingly, in themethod of adjusting a frequency described in WO2008/062639 A1, in orderto obtain desired frequency characteristics, it has been necessary toform a reforming portion near the boundary of media with a high degreeof accuracy.

However, in the method of adjusting a frequency described inWO2008/062639 A1, it is necessary to keep the melting point of thereforming medium layer 102 c low or to select material that easilybecomes diffuse, as the reforming medium layer 102 c, so that mediaother than the reforming medium layer 102 c and the IDT electrode do notbecome diffuse, and there has been a problem that device design islargely constrained. In addition, in order to reform the reformingmedium layer 102 c or diffuse metal included in the reforming mediumlayer 102 c into the surrounding media 102 a and 102 b, it is necessaryto cause melting and a solidification action to occur near the meltingpoint of the reforming medium layer 102 c. However, if local heating isperformed using a focused laser so as to cause the melting and thesolidification to occur, the film quality or the diffusion state of thereforming layer becomes non-uniform, and it has been very difficult toadjust a frequency with a high degree of accuracy. Accordingly, it hasbeen difficult to obtain a boundary acoustic wave device includingdesired frequency characteristics.

SUMMARY OF THE INVENTION

Accordingly, preferred embodiments of the present invention provide amanufacturing method for a boundary acoustic wave device, which iscapable of easily and certainly obtaining the boundary acoustic wavedevice including desired frequency characteristics, and the boundaryacoustic wave device.

According to a preferred embodiment of the present invention, a methodof manufacturing a boundary acoustic wave device includes a process forpreparing a laminated body including a first medium made of apiezoelectric substance, a second medium made of a dielectric that islaminated on the first medium, and an IDT electrode that is disposed atan interface between the first medium and the second medium, and aprocess for implanting ions or atoms from an outer side portion of thesecond medium and adjusting a frequency.

In a specific aspect of the manufacturing method for the boundaryacoustic wave device according to a preferred embodiment of the presentinvention, when the wavelength of a boundary acoustic wave is λ, ionsare distributed in a region located within about one λ above and belowthe interface owing to the ion implantation. In this case, since thepropagation energy of the boundary acoustic wave is concentrated intothe region located within about one λ above and below the interface, itis possible to adjust the frequency characteristics of the boundaryacoustic wave device with a higher degree of accuracy.

In another specific aspect of the manufacturing method for the boundaryacoustic wave device according to a preferred embodiment of the presentinvention, at the time of the ion implantation, ions of atoms whoseatomic weights are greater than or equal to Li are implanted. In thiscase, since the ions of atoms whose atomic weights are large areimplanted, the implanted ions are difficult to be extracted.Accordingly, it is possible to provide the boundary acoustic wave devicewhose frequency characteristics are stable.

In another specific aspect of the manufacturing method for the boundaryacoustic wave device according to a preferred embodiment of the presentinvention, in the process for preparing the laminated body, the secondmedium is formed so that the thickness of the second medium becomes athickness causing elastic waves excited by the IDT electrode to mainlyinclude not boundary acoustic waves but surface acoustic waves.Furthermore, after the ion implantation process, a third mediumincluding the same dielectric material as the second medium or adielectric material different from the second medium is further formedon the second medium so that elastic waves excited by the IDT electrodemainly include boundary acoustic waves. In this case, since thethickness of the second medium is thin, it is possible to performadjustment of frequency using small ion implantation energy.Accordingly, it is possible to perform adjustment of frequency usinglow-priced equipment. In addition, since the thickness of the secondmedium is thin, it is possible to easily implant ions whose atomicweights are large. When ions of atoms whose atomic weights are large areimplanted, frequency characteristics greatly change. Accordingly, it ispossible to reduce the amount of ions implantation with respect to theamount of adjustment of frequency, and it is possible to easily performadjustment of frequency.

According to preferred embodiments of the present invention, theboundary acoustic wave device includes a first medium made of apiezoelectric body, a second medium made of a dielectric that islaminated on the first medium, and an IDT electrode that is disposed atan interface between the first medium and the second medium, and thefirst medium and/or the second medium includes an ion or atomimplantation region.

In a specific aspect of the boundary acoustic wave device according toanother preferred embodiment of the present invention, when thewavelength of a boundary acoustic wave is λ, a region located withinabout one λ above and below the interface is the ion or atomimplantation region. In this case, it is possible to adjust thefrequency characteristics of the boundary acoustic wave device with ahigher degree of accuracy.

In another specific aspect of the boundary acoustic wave deviceaccording to a preferred embodiment of the present invention, the ionsare ions of atoms whose atomic weights are greater than or equal to Li.In this case, since the implanted ions are difficult to be extracted, itis possible to provide the boundary acoustic wave device whose frequencycharacteristics are stable.

In another specific aspect of the boundary acoustic wave deviceaccording to a preferred embodiment of the present invention, thethickness of the second medium is a thickness causing elastic wavesexcited by the IDT electrode to mainly include not boundary acousticwaves but surface acoustic waves, and further includes a third medium,which is laminated on the second medium and includes the same dielectricmaterial as the second medium or dielectric material different from thesecond medium. The third medium is arranged so that elastic wavesexcited by the IDT electrode mainly include boundary acoustic waves. Inthis case, since the thickness of the second medium is thin, less ionimplantation energy is necessary. Accordingly, since it is possible toperform adjustment of frequency using low-priced equipment, it ispossible to reduce the manufacturing cost of the boundary acoustic wavedevice. In addition, it is possible to easily implant ions whose atomicweights are large. Therefore, it is possible to easily performadjustment of frequency.

According to the manufacturing method for the boundary acoustic wavedevice and the boundary acoustic wave device according to variouspreferred embodiments of the present invention, at a stage where thelaminated body in which the IDT electrode is disposed at the interfacebetween the first and second media is obtained, it is possible to adjusta frequency owing to the ion implantation. Accordingly, after thecompletion of the boundary acoustic wave device, it is possible toeasily adjust the frequency so as to obtain target frequencycharacteristics. Alternatively, after the second medium has been formedso that elastic waves excited by the IDT electrode mainly includesurface acoustic waves, it is possible to easily adjust the frequencyowing to the ion implantation.

In addition, since the frequency is adjusted owing to the ionimplantation, it is possible to certainly distribute ions to a portionnear the interface between the first and second media. Therefore, it ispossible to perform adjustment of frequency with a high degree ofaccuracy. According to various preferred embodiments of the presentinvention, it is possible to easily and certainly provide the boundaryacoustic wave device including target frequency characteristics.

The above and other elements, features, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of the preferred embodiments withreference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an elevational cross-sectional view for explaining a processin which a frequency is adjusted owing to ion implantation in amanufacturing method for a boundary acoustic wave device according to apreferred embodiment of the present invention, and FIG. 1B is apartially enlarged elevational cross-sectional view for explaining anelectrode structure of the boundary acoustic wave device.

FIG. 2 is a pattern diagram illustrating vibration displacementdistribution of boundary acoustic waves propagating through the boundaryacoustic wave device according to the first preferred embodiment of thepresent invention.

FIG. 3 is a diagram illustrating a relationship between the amount offrequency change and the amount of ion implantation in the manufacturingmethod for the boundary acoustic wave device according to the firstpreferred embodiment of the present invention.

FIG. 4 is a diagram illustrating a relationship between the amount ofimpedance change and the amount of ion implantation in the manufacturingmethod for the boundary acoustic wave device according to the firstpreferred embodiment of the present invention.

FIG. 5A is a diagram illustrating impedance characteristics before ionimplantation is performed and after ion implantation has been performedwith the amount of ion implantation being about 1×10¹⁵ atom/cm², in themanufacturing method for the boundary acoustic wave device according tothe first preferred embodiment, and FIG. 5B is a diagram illustratingphase characteristics before ion implantation is performed and after ionimplantation has been performed with the amount of ion implantationbeing about 1×10¹⁵ atom/cm², in the manufacturing method for theboundary acoustic wave device according to the first preferredembodiment of the present invention.

FIG. 6A is a diagram illustrating impedance characteristics before ionimplantation is performed and after ion implantation has been performedwith the amount of ion implantation being about 3×10¹⁵ atom/cm², in themanufacturing method for the boundary acoustic wave device according tothe first preferred embodiment, and FIG. 6B is a diagram illustratingphase characteristics before ion implantation is performed and after ionimplantation has been performed with the amount of ion implantationbeing about 3×10¹⁵ atom/cm², in the manufacturing method for theboundary acoustic wave device according to the first preferredembodiment of the present invention.

FIG. 7A is a diagram illustrating impedance characteristics before ionimplantation is performed and after ion implantation has been performedwith the amount of ion implantation being about 1×10¹⁶ atom/cm², in themanufacturing method for the boundary acoustic wave device according tothe first preferred embodiment, and FIG. 7B is a diagram illustratingphase characteristics before ion implantation is performed and after ionimplantation has been performed with the amount of ion implantationbeing about 1×10¹⁶ atom/cm², in the manufacturing method for theboundary acoustic wave device according to the first preferredembodiment of the present invention.

FIG. 8A is a diagram illustrating impedance characteristics before ionimplantation is performed and after ion implantation has been performedwith the amount of ion implantation being about 5×10¹⁶ atom/cm², in themanufacturing method for the boundary acoustic wave device according tothe first preferred embodiment, and FIG. 8B is a diagram illustratingphase characteristics before ion implantation is performed and after ionimplantation has been performed with the amount of ion implantationbeing about 5×10¹⁶ atom/cm², in the manufacturing method for theboundary acoustic wave device according to the first preferredembodiment of the present invention.

FIG. 9 is a diagram illustrating a relationship between the amount ofion implantation and the amount of frequency change when He⁺ ions areimplanted.

FIG. 10 is a diagram illustrating a relationship between the amount ofion implantation and the amount of frequency change when B⁺ ions areimplanted.

FIGS. 11A and 11B are individual schematic elevational cross-sectionalviews illustrating a manufacturing method for a boundary acoustic wavedevice according to another preferred embodiment of the presentinvention.

FIG. 12 is a schematic elevational cross-sectional view for explaining afrequency adjusting method for a boundary acoustic wave device of therelated art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, specific preferred embodiments of the present inventionwill be described with reference to drawings, thereby clarifying thepresent invention.

A manufacturing method for a boundary acoustic wave device according toa preferred embodiment of the present invention will be described withreference to FIGS. 1A and 1B. First, a first medium 1 illustrated inFIG. 1A is prepared. As the first medium 1, in the present preferredembodiment, a substantially 25-degree Y-cut LiNbO₃ substrate ispreferably used, for example. As a matter of fact, the first medium 1may also be formed using another piezoelectric substance. As such apiezoelectric substance, LiNbO₃ whose Cut-Angles are other than theabove-mentioned Cut-Angles, LiTaO₃, crystal, piezoelectric ceramics, orother suitable material may be used, for example. In addition, as thepiezoelectric body, a laminated body including a piezoelectric film anda supporting substrate may be used in which the piezoelectric film isprovided on the supporting substrate such as a glass substrate, an Sisubstrate, a sapphire substrate, or other suitable substrate, forexample.

On the first medium 1, an IDT electrode 2 and a reflector notillustrated are formed. A forming method for the IDT electrode 2 is notlimited to a specific method, and a thin-film forming method such asvapor deposition, sputtering, or other suitable method may be used, forexample. In the present preferred embodiment, as the IDT electrode 2, anIDT electrode is formed that includes laminated metal films enlarged andillustrated in FIG. 1B. In the laminated metal films, starting from thebottom, these metal films, namely, a Ti film 2 a, a Pt film 2 b, a TiO₂film 2 c, an AlCu film 2 d, a TiO₂ film 2 e, a Pt film 2 f, and an NiCrfilm 2 g, are laminated in that order. The film thicknesses ofindividual metal films are as follows. In addition, the AlCu film isalloy whose main component is Al and that includes Cu.

NiCr/Pt/TiO₂/AlCu/TiO₂/Pt/Ti=about 10/22/20/225/20/36/10 (the unitthereof is nm).

The pitch of electrode fingers included in the IDT electrode and thereflector is, for example, about 2.01 μm, and the pitch of electrodefingers substantially corresponds to the wave length λ of a boundaryacoustic wave at a response frequency of the boundary acoustic wavedevice. The intersecting width of the IDT electrode is about 28.7 μm,the number of electrode fingers of the IDT electrode is about 227, thenumbers of electrode fingers of the reflectors disposed on both sides ofthe IDT electrode in a boundary acoustic wave propagation direction areindividually about 31, and the ratio relative to the wavelength of theelectrode finger width of each of the IDT electrodes and the reflectoris about 0.25, for example.

After the above-mentioned IDT electrode 2 has been formed, an SiO₂ film3 is formed. The film formation of the SiO₂ film 3 is performed using anarbitrary method such as sputtering or other suitable method, forexample. The film thickness of the SiO₂ film 3 is preferably about 1213nm, for example.

Next, on the SiO₂ film 3, the film of an SiN film 4 is formed preferablyby sputtering. The film thickness of the SiN film 4 is preferably about400 nm, for example.

The above-mentioned SiO₂ film 3 and SiN film 4 correspond to the secondmedium.

In addition, while, in FIG. 1A, not illustrated, an electrode extractionpad is formed in a process for forming the IDT electrode 2, so as to beelectrically connected to the IDT electrode 2. In addition, using aphotolithographic method, the SiO₂ film 3 and the SiN film 4 aresubjected to patterning so that the SiO₂ film 3 and the SiN film 4 onthe electrode extraction pad are removed.

In the laminated body obtained as described above, the frequencycharacteristics thereof are measured. In the present preferredembodiment, one port-type boundary acoustic wave resonator is configuredin which the reflectors are provided on both sides of the IDT electrode2. Accordingly, as the frequency characteristics, a resonancecharacteristic, an impedance characteristic and/or an impedance Smithchart, and the like are measured. When the time comes, as illustrated byan arrow in FIG. 1A, metal ions are implanted into the second mediumfrom the outer side portion thereof, specifically, from above the SiNfilm using IMX-3500RS (a medium current density ion implantationapparatus) manufactured by ULVAC, Inc. In the present preferredembodiment, as the metal ions, Li ions are preferably implanted. In theion implantation, it is possible to certainly distribute ions in thevicinity of the interface between the first medium 1 and the secondmedium including the SiO₂ film 3 and the SiN film 4. In such a way asdescribed above, it is possible to form a region 5 into which ions areimplanted, as illustrated in FIG. 1A. When the region 5 into which ionsare implanted is formed, the propagation behaviors of the boundaryacoustic waves excited by the IDT electrode 2 change, and the frequencycharacteristics thereof change.

In such a way as described above, it is possible to adjust the frequencycharacteristics owing to the ion implantation. A specific adjustment offrequency result due to the ion implantation will be described on thebasis of a specific experimental example, later.

In the present preferred embodiment, as described above, only byperforming the ion implantation from the outside after the laminatedbody has been obtained, it is possible to easily and certainly adjustthe frequency of the boundary acoustic wave device.

In addition, while, in the above-mentioned preferred embodiment, thesecond medium is preferably formed using the SiO₂ film 3 and the SiNfilm 4, the second medium may also be configured using one type ofdielectric. In addition, the materials used for configuring the secondmedium are not limited to SiO₂ or SiN, and various types of dielectricsmay also be used.

Next, a specific experimental example will be described.

After the laminated body illustrated in FIG. 1A has been prepared, Li⁺ions are implanted into the second medium from the top surface thereofwith implantation energy being about 200 KeV, using IMX-3500RS (a mediumcurrent density ion implantation apparatus) manufactured by ULVAC, Inc,and hence frequency adjustment is performed. In this case, the amount ofion implantation is variously changed within a range from about 1×10¹⁴atom/cm² to about 1×10¹⁷ atom/cm², for example, and frequency adjustmentis performed. The results are illustrated in FIGS. 3 and 4. FIG. 3illustrates a relationship between the amount of ion implantation andthe amount of frequency change. In FIG. 3, “◯” indicates a resonancefrequency Fr, and “x” indicates an anti-resonant frequency Fa.

As is clear from FIG. 3, it is understood that the resonance frequencyand the anti-resonant frequency decrease with an increase in the amountof ion implantation. Namely, it is understood that there are negativecorrelations between the amount of ion implantation and the resonancefrequency and anti-resonant frequency. Accordingly, by changing theamount of ion implantation, it is possible to adjust a frequency so thatthe frequency becomes a target frequency.

In addition, FIG. 4 illustrates a relationship between the amount of ionimplantation described above and the amount of impedance change. Theamount of impedance change indicates the amount of change in a ratio ofan impedance at the resonance frequency to an impedance at theanti-resonant frequency. As is clear from FIG. 4, it is understood thatan impedance ratio is less changed even if ion implantation isperformed.

Specific examples of the changes of the impedance characteristics andthe phase characteristics when the above-mentioned amount of ionimplantation is changed will be specifically illustrated in FIGS. 5A to8B.

FIG. 5A, FIG. 6A, FIG. 7A, and FIG. 8A are diagrams individuallyillustrating impedance characteristics before and after ionimplantation, and FIG. 5B, FIG. 6B, FIG. 7B, and FIG. 8B are diagramsindividually illustrating phase characteristics before and after ionimplantation. In FIG. 5A to FIG. 8B, solid lines indicatecharacteristics after ion implantation, and dashed lines indicatecharacteristics before ion implantation.

In addition, the amounts of ion implantation are as follows.

In FIGS. 5A and 5B, the amounts of ion implantation are about 1×10¹⁵atom/cm², in FIGS. 6A and 6B, the amounts of ion implantation are about3×10¹⁵ atom/cm², in FIGS. 7A and 7B, the amounts of ion implantation areabout 1×10¹⁶ atom/cm², and in FIGS. 8A and 8B, the amounts of ionimplantation are about 5×10¹⁶ atom/cm².

The results in FIG. 5A to 8B correspond to the result summarized inFIGS. 3 and 4.

In addition, since the amounts of changes in the impedance change andthe phase change are small in a case in which the amounts of ionimplantation are about 1×10¹⁴ atom/cm² in FIGS. 3 and 4, theillustration thereof is omitted.

As is clear from the above-mentioned experimental examples, it isunderstood that, by controlling the amount of ion implantation, it ispossible to adjust the resonance frequency and the anti-resonantfrequency with a high degree of accuracy.

In the above-mentioned preferred embodiment, owing to the ionimplantation, it is possible to adjust the frequency with a high degreeof accuracy. This may be because, owing to the ion implantation, it ispossible to distribute ions in the vicinity of the boundary of the firstand second media, as in the region 5 illustrated in FIG. 1A. Asillustrated in FIG. 2, on the basis of the vibration displacementdistribution of an elastic wave of the above-mentioned boundary acousticwave device, the energy of the elastic wave is concentrated into aportion near the boundary of the first medium and the second medium. Thedepth 0.0 of a vertical axis in FIG. 2 indicates the boundary betweenthe first medium and the second medium, a minus direction corresponds toa first medium side, and a plus direction corresponds to a second mediumside. In FIG. 2, U1 indicates displacement distribution in a propagationdirection, U2 indicates a vibration displacement component in adirection vertical to the propagation direction and parallel to theboundary, and U3 indicates a vibration displacement component in adirection vertical to the propagation direction and the boundary. Inaddition, in many cases, a boundary acoustic wave propagates as onepropagation mode with the vibration displacement components of U1, U2,and U3 being coupled to one another. As is clear from FIG. 2, when thewave length of the boundary acoustic wave is λ, the energy of theelastic wave is distributed within a range where a depth is about0.0±1.0λ. In addition, most of the energy of the elastic wave isconcentrated into a range where a depth is about 0.0±0.70λ, and theenergy thereof gradually increases as it gets closer to the depth 0.0,namely, the boundary of the first medium and the second medium. Inaddition, from among SiO₂ and SiN included in the second medium, on theinner side of SiO₂ whose acoustic velocity is lower, vibrationdisplacement rapidly increases. Accordingly, by distributing ions in aportion located within about one λ above and below the above-mentionedboundary owing to ion implantation, it is possible to change thepropagation behavior of the elastic wave, and it is possible to moregreatly change the propagation behavior of the elastic wave within arange of about 0.7λ above and below the boundary. Namely, it is possibleto greatly change the frequency. Furthermore, it is understood that,when the medium 2 is configured by laminating a film whose acousticvelocity is high and a film whose acoustic velocity is low, much energyis distributed in the film whose acoustic velocity is low. Accordingly,it is possible to adjust the frequency to an even greater extent.

When the distribution of Li ions in the above-mentioned preferredembodiment is analyzed on the basis of an LSS theory (for example,non-patent literature J. Lindhard et al.; “Range Concepts and Heavy IonRanges,” Mat. Fys. Medd. Dan. Vid. Selsk., Vol. 33, p. 1-39, 1963), itis confirmed that Li is distributed with a position in the film of theSiO₂ film 3 as the center thereof, the position being located about 770nm (about 0.38λ because of λ=2.01 μm) away from the boundary of theLiNbO₃ substrate 1 and the SiO₂ film 3.

In the above-mentioned experimental example, the frequencycharacteristics greatly change owing to the ion implantation. This maybe because the region 5 illustrated in FIG. 1A is formed in a region ofabout 0.38λ located near the boundary of the first medium and the secondmedium, in which the energy of the elastic wave is distributed. For thatmatter, this may be because the region 5 is formed within SiO₂ locatednear the boundary in the medium 2 configured by laminating SiN whoseacoustic velocity is high and SiO₂ whose acoustic velocity is low, andhence a greater frequency change is obtained.

In addition, in the above-mentioned preferred embodiment, since theimplantation energy is reduced to about 200 keV owing to the restrictionof the ion implantation apparatus used for ion implantation, thethickness of the SiN film included in the second medium is about 400 nm.Therefore, vibration energy is slightly distributed on the SiN surface.Therefore, in the same procedure as in the above-mentioned preferredembodiment, a sample subjected to ion implantation and a sample notsubjected to ion implantation are prepared, the film of SiN is formed tobe about 1600 nm in thickness on the SiN film whose thickness is about400 nm after ion implantation, thereby forming a complete boundary wavewhose vibration energy is not distributed on the surface, and afrequency difference between the sample subjected to ion implantationand the sample not subjected to ion implantation is compared. As aresult, the same frequency change as the frequency changed owing to theion implantation is confirmed. On the basis of these studies, it may bejudged that the frequency change due to the ion implantation does notresult from the change of thickness on the surface of the SiN film ofabout 400 nm, due to ion milling but results from the propagationbehavior of the boundary wave, changed by distributing Li in a portionlocated near the boundary using ion implantation and altering theelastic constant of the medium 2.

Accordingly, in various preferred embodiments of the present invention,it is desirable that it is only necessary to perform ion implantation onthe region located within a range of about one λ above and below theabove-mentioned boundary, in the first medium and/or second medium. Whenions are implanted into a region ranging from about 0.7λ to about 1λabove and below the boundary, a great frequency change is not obtained.However, it may be expected that the frequency moderately changes withrespect to the amount of ion implantation. Accordingly, it is suitablefor the fine adjustment of the frequency. On the other hand, whenmanufacturing variations are large and frequency adjustment is performedso as to greatly change a frequency, it is desirable that ions areimplanted into the region located within a range of about 0.7λ above andbelow the boundary. Accordingly, it is possible to greatly change thefrequency.

Furthermore, it is also desirable that the first medium or the secondmedium has a laminate structure. Consequently, by implanting ions into amedium layer located near the boundary in which energy steeplyincreases, it is possible to change the frequency to a greater extent.Accordingly, it is also possible to deal with a case in whichmanufacturing variations are large, which is desirable.

In the same way as the above-mentioned experimental example, frequencyadjustment is tried. In this regard, however, ionic species to beimplanted are He⁺ and B⁺ ions. The results are illustrated in FIGS. 9and 10. FIG. 9 illustrates a relationship between the amount of ionimplantation and the amount of frequency change when He ions areimplanted, and FIG. 10 illustrates a relationship between the amount ofion implantation and the amount of frequency change when B⁺ ions areimplanted. Also in FIGS. 9 and 10, in the same way as in FIG. 3, “◯”indicates the resonance frequency Fr, and “x” indicates theanti-resonant frequency Fa.

As is clear from FIG. 9, when He⁺ ions are implanted, there is a poorcorrelation between the amount of implantation and the amount offrequency change while the frequency characteristics change. This may bebecause, in the case of He⁺ ions, implanted ion tends to be easilyextracted and hence the effect of implantation varies. Accordingly,optimally, it is desirable that ions of atoms whose atomic weights aregreater than He, namely, ions of Li or ions of atoms whose atomicweights are greater than Li, are implanted. As a matter of fact, as isclear from FIG. 9, when He⁺ ions are implanted, it is also possible tochange the resonance frequency and the anti-resonant frequency.Accordingly, it is also possible to adjust the frequencycharacteristics.

In the same way, as illustrated in FIG. 10, when B⁺ ions are implanted,there is a minimal change in the correlation between the amount ofimplantation and the amount of frequency change. However, it is alsopossible to adjust the frequency characteristics. Compared with Li, Bhas a large atomic weight. Therefore, even if B⁺ ions are implanted withabout the same 200 keV as Li, an implantation depth becomes shallow andthe region 5 illustrated in FIG. 1A is formed with a position as thecenter thereof, the position being located about 0.45λ away from theboundary of the first medium and the second medium (0.45λ corresponds toa position within SiO₂ because the thickness of SiO₂ is about 0.6λ).Consequently, it may be considered that the amount of frequency changebecomes small. In addition, since the atomic weight of B is large, thechange of the density of the region 5 due to the implanted ions becomeslarge, and the produced distortion of a surrounding film becomes large.Therefore, when the same amount of ions is implanted into a same depth,the amount of change in B becomes large, compared with Li.

FIGS. 11A and 11B are individual schematic elevational cross-sectionalviews illustrating a manufacturing method for a boundary acoustic wavedevice according to another preferred embodiment of the presentinvention. In the second preferred embodiment, as illustrated in FIG.11A, first, a laminated body 12 is prepared in which a first medium 1,an IDT electrode 2, and a second medium 11 are laminated in this order.The first medium 1, the IDT electrode 2, and the second medium 11 may beconfigured using the same materials as in the above-mentioned preferredembodiment. In this regard, however, in the present preferredembodiment, the film of the second medium 11 is preferably formed sothat elastic waves excited when the IDT electrode 2 is excited mainlyinclude not boundary acoustic waves but surface acoustic waves.

In this stage, ion implantation is performed as illustrated by an arrow,the region 5 is formed, and frequency adjustment is performed. Namely,in the stage of the laminated body 12 that is a surface acoustic wavedevice, frequency adjustment is performed using ion implantation. Asdescribed above, frequency adjustment may be performed in the stage ofthe surface acoustic wave device.

Next, after frequency adjustment, a third medium 13 is laminated on thesecond medium 11. The third medium 13 may be formed using the samedielectric material as the second medium 11 or formed using dielectricmaterial different from the second medium 11.

In this regard, however, the third medium 13 is laminated so that, bylaminating the third medium 13, elastic waves excited by applying avoltage to the IDT electrode 2 are boundary acoustic waves. Namely, thetotal thickness of the second medium 11 and the third medium 13 is setso that the excited elastic waves are boundary acoustic waves, and hencethe boundary acoustic wave device is obtained.

In the present preferred embodiment, in the stage of the laminated body12 that is a surface acoustic wave device, frequency adjustment isperformed. Accordingly, also in the present preferred embodiment, whilethe IDT electrode 2 is embedded between the first and second media, itis possible to perform frequency adjustment with a high degree ofaccuracy, by performing the ion implantation from the outside.Accordingly, also in the present preferred embodiment, it is possible toeasily and certainly obtain the boundary acoustic wave device havingultimate target frequency characteristics.

In addition, the effects and advantages of various preferred embodimentsof the present invention are due to the change in the density or theelastic constant of a medium in a region in which the energy of boundarywaves are distributed, the change being caused by the change in thedensity or the distortion due to implanted particles. Accordingly, withrespect to the effects and advantages of various preferred embodimentsof the present invention, the implanted particles are not limited toions. In addition, even if electrons are supplied to accelerated ionsusing a neutralizer or the like to neutralize ions, and atoms obtainedby neutralization are implanted, the same advantageous effects may beobtained.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing from the scopeand spirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

What is claimed is:
 1. A manufacturing method for a boundary acousticwave device, comprising: a step of preparing a laminated body includinga first medium made of a piezoelectric substance, a second medium madeof a dielectric that is laminated directly on the first medium, and anIDT electrode that is disposed at an interface between the first mediumand the second medium; and a step of implanting ions or atoms from anouter portion of the second medium in a thickness direction of thesecond medium so as to adjust a frequency of the boundary acoustic wavedevice, forming the boundary acoustic wave device; wherein a wavelengthof a boundary acoustic wave is λ, and the ions or atoms are distributedin a region located within about 1λ above and below the interface in thestep of implanting ions or atoms.
 2. The manufacturing method for theboundary acoustic wave device according to claim 1, wherein the ions oratoms are distributed in a region located within about 0.7λ to about 1λabove and below the interface in the step of implanting ions or atoms.3. The manufacturing method for the boundary acoustic wave deviceaccording to claim 1, wherein during the step of implanting ions oratoms, ions or atoms whose atomic weights are greater than or equal toLi are implanted.
 4. The manufacturing method for the boundary acousticwave device according to claim 1, wherein in the step of preparing thelaminated body, the second medium is formed so that a thickness of thesecond medium becomes a thickness causing elastic waves excited by theIDT electrode to mainly include not boundary acoustic waves but surfaceacoustic waves, wherein the manufacturing method further comprises: astep of forming a third medium including the same dielectric material asthe second medium or dielectric material different from the secondmedium, on the second medium so that elastic waves excited by the IDTelectrode mainly include boundary acoustic waves, after the step ofimplanting ions or atoms.